Can a landlord keep your deposit for painting rooms?

Deposits can not be used for reasonable wear and tear. They are for damage which typically does not include just painting. You should consult an attorney or a small claims advisor about bringing a small claims action for return of your deposit.

Security deposits can typically with withheld by the landlord for overdue past rent and damages/repairs to the apartment as a result of the tenant. A paint job, especially one done by you while living there as opposed to one the landlord was forced to do after you moved out due to some damage issue to the walls for example, does not sound proper to withhold your money for. But there may be more to the story, including whether the lease allowed such "alterations" to the property, even if positive/aesthetic. Either way, if you all are in dispute, you can take it to small claims court

In Idaho, the landlord is required to give you a written itemization of the charges for necessary repairs, cleaning or damage in order to keep any (or all) of your deposit. Failure to abide by the law could result in triple damages. Talk to an attorney with experience in landlord-tenant law.